1. Field of the Invention
The present invention relates to a vibration actuator which obtains a driving force from vibration waves.
2. Related Background Art
As an ultrasonic motor using ultrasonic vibrations, a standing wave type linear ultrasonic monitor using standing waves produced by an elastic vibration member has been proposed. In this motor, voltages having a specific frequency are applied to a vibrator bonded to the elastic vibration member to excite bending vibrations and longitudinal vibrations in the elastic vibration member, and a driving force is obtained through a motion extraction portion disposed on the elastic vibration member.
The driving principle of the standing wave type linear ultrasonic monitor will be described next with reference to FIGS. 6A and 6B.
Referring to FIGS. 6A and 6B, this motor includes a movable member 51, a vibration member 52 made of an elastic member, motion extraction portions 52a and 52b integrally formed with the elastic vibration member 52, a piezoelectric member 53 for exciting longitudinal vibrations and bending vibrations in the elastic vibration member 52, and electrode films 53a, 53b, and 53c through which voltages having a specific frequency are applied to the piezoelectric member 53. The elastic vibration member 52 and the piezoelectric member 53 are integrally formed by an adhesive etc., and thereby a vibration element is formed. Note that a voltage application circuit is not shown.
In the above arrangement, when voltages having a specific frequency and a 90.degree. phase difference are respectively applied to the electrode films 53a and 53b, the piezoelectric member 53 repeats expansion-shrinkage at the specific frequency. Upon expansion-shrinkage of the piezoelectric member 53, longitudinal vibrations and bending vibrations are excited in the elastic vibration member 52, and the motion extraction portions 52a and 52b rotate in the same direction to perform elliptic motions. When the movable member 51 is pressed against the motion extraction portions 52a and 52b, the movable member 51 moves in, e.g., the direction indicated by the arrow in FIG. 6A.
Referring to FIG. 6A, the bending vibrations are of the fourth-order mode, and the longitudinal vibrations are of the first-order mode. However, vibration modes to be used are not limited to these modes as long as a driving force can be obtained.
In the above prior art, since leads for voltage application are directly soldered to a surface of a piezoelectric member, it is difficult to laminate piezoelectric members because of a connection structure formed by soldering. In addition, it is difficult to decrease the driving voltage or increase the driving force by increasing the amplitudes of elliptic motions using a laminated structure.
Various forms of ultrasonic monitors have been proposed from the viewpoint of vibration elements for producing vibrations. FIGS. 13A and 13B show an ultrasonic motor having a vibration element V in the form of a flat plate, which is disclosed in, e.g., Japanese Laid-Open Patent Application Nos. 63-290176, 63-294269, 1-110070, and 1-110071.
This vibration element V is designed such that bending vibrations of the B mode in FIG. 14B and longitudinal vibrations of the L mode in FIG. 14C are synthesized to cause driving pieces 103a and 103b disposed at the positions of antinodes of bending vibrations of the B mode to perform elliptic motions as shown in FIG. 14A.
The vibration element V is disposed between an upper wall 109 and a lower wall 110. Roller bars 106a 106b respectively having rollers 108a and 108b on their lower ends extend from the upper wall 109.
A support member 104 of the vibration element V is fitted in a hole portion in the lower wall 110. The vibration element V is elastically supported by a spring 107 disposed in the hole portion. The upward movement of the vibration element V is restricted by the rollers 108a and 108b. The driving pieces 103a and 103b of the vibration element V are pressed against a movable member 105 which is allowed to move in the x direction. The movable member 105 is moved in the x direction by a frictional driving force produced by elliptic motions of the driving pieces 103a and 103b.
As shown in FIG. 13B, in the vibration element V for producing vibrations of the B and L modes, pairs of driving piezoelectric elements 101a and 101b, and 101c and 101d which respectively constitute vibrators are respectively bonded to the upper and lower surfaces of a ground electrode plate 102 in the form of a rectangular, flat plate. As shown in FIG. 15A, these driving piezoelectric elements 101a, 101b, 101c, and 101d are polarized in the direction of thickness. The polarization directions are indicated by the arrows in FIG. 15A.
As shown in FIG. 15A, a B input is applied to the driving piezoelectric elements 101a and 101d, which are polarized in the direction of thickness, and an A input as an electric field having a reverse polarity to the B input is applied to the driving piezoelectric elements 101b and 101c. As a result, as shown in FIG. 15B, for example, the driving piezoelectric elements 101a and 101d, to which the B input as an electric field is applied, expand, and the driving piezoelectric elements 101b and 101c, to which the A input as an electric field is applied, shrink. When this expansion-shrinkage is repeated, bending vibrations of the B mode are produced.
When electric fields having the same polarity are applied to all the driving piezoelectric elements 101a, 101b, 101c, and 101d in FIG. 15A, they expand/shrink, as shown in FIG. 15C or 15D, thereby producing longitudinal vibrations of the L mode.
If the two vibration modes, i.e., the B and L modes, of the vibration element V have almost equal resonant frequencies, both the vibration modes are simultaneously produced by exciting the vibrators at the resonance frequency. As shown in FIG. 16, when sin and cos waves having a 90.degree. phase difference are applied, as the A and B inputs, to the driving piezoelectric elements to shift the phase of vibrations of the B mode from that of the A mode by 90.degree., the driving pieces perform elliptic motions. By bringing the driving pieces into frictional contact with the movable member, the movable member can be driven. Referring to FIG. 16, ".smallcircle." indicates shrinkage; ".circle-solid.", expansion; ".diamond.", bending; and ".diamond-solid.", bending.
In the above conventional vibration device, actual measurement of vibrations of the vibration element V is the only way to determine whether the movable member 105 is properly driven, i.e., whether vibrations of the B and L modes are properly produced. In spite of the fact that the torque and efficiency of an ultrasonic monitor greatly change depending on a frequency to be input, frequency control for optimal characteristics is performed for each operation by experimentally changing the frequency.
Vibrations of the B and L modes are produced at resonant frequencies as values (first-order, second-order, third-order, . . . ) based on the natural frequencies of the vibrators in the respective modes. However, resonance points shift depending on environmental conditions such as temperature and humidity, so that the same driving force cannot be obtained from vibrations produced at the same frequency.